1. Field of the Invention
This invention relates to a method of injection compression molding and an apparatus for performing the same.
2. Description of the Prior Art
In injection molding, molten resin is injected into a cavity of a mold firmly held by the mold structure, an injection cylinder applies pressure through the resin in a spue or runner in an operation called applying the holding pressure until the resin at a narrow gate at the spue or runner cures to thereby prevent counter flow of the resin in the cavity, and after resin at the gate cures, the heat of resin in the cavity is removed through the mold to cool the resin, thus obtaining the molded resin product.
The density of the molten resin usually is smaller than that of the solid resin and the resin diminishes in volume as the resin cures, thereby causing shrinkage of the resin in the mold. For example, in a case where a product 100 mm in diameter and 10 mm thick has density of 1.17 in the molten state and 1.20 in the solid state, its volume becomes 76.57 cc when it reaches ambient temperature after the resin cures from the molten state where its volume is 78.54 cc, i.e. equal to that of the mold cavity. Assuming that the reduced volume is due only to shrinkage in the thickness direction, the finished product is 9.75 mm, which is less than the desired thickness by 0.25 mm.
As a countermeasure for overcoming mold shrinkage, a method has been studied which supplies an excess of resin to the mold until it slightly opens at the parting line. A method for controlling the amount of opening the mold (cited in Japanese Patent Laid-Open No. Sho-50-39851), and the Rolinx method of over filling ("new concept in injection molding, Rolinx process extended application of plastics" Plastics, 30, 330, April (1965)), have been proposed. Also, a method has been proposed which has a small hydraulic piston in a cylinder in the mold to cause a core to move into the mold cavity, or a piston-cylinder device is used for an ejector, so that resin is injected into an intentionally enlarged cavity and the hydraulic piston moves forwardly to reduce the size of the cavity, thereby producing a molded product of the predetermined thickness, this method being known as the Micro Molder method (H. Holt: "New technique in shrinkage control" SPE J. P519, June (1964)).
Needless to say, it is an elemental concept to design a larger cavity than that of the finished product in anticipation of the shrinkage, but for a product with a larger thickness or with different thicknesses, resin leaves the mold at the thick wall portion, which makes such a design impossible in practice. Hence, a trial and error method must be used.
The aforesaid over filling method is recognized as requiring a high injection pressure and also as having limited shrinkage compensation effect in a thin wall portion where less shrinkage occurs when the product has different thicknesses. In the Micro Molder Method, a cylinder ram advances following the shrinkage of the resin in the mold, so that while the surface of the product at the moving core side will be finished with accuracy, the opposite surface may be finished with insufficient accuracy.
In the light of the above, an injection compression molding method for obtaining compression of the resin by use of a mold clamping force has been proposed by ENGEL CO. (LUDWIG ENGEL KG MACHINEN FABRIK, A-4311 SCHWERTERG AUSTRIA). This method uses a toggle joint which is not fully extended during the injection process and which is extended to the full extent during the compression process, so that the toggle system mold clamping force acts as the compression pressure.
The compression by the use of the toggle joint is deficient in that there is no control of the compression pressure. The necessity of control for compression pressure will be explained by reference to the PVT curve of FIG. 1 showing the relationship of the pressure applied to the resin, the specific volume of the resin and the temperature of the resin. The abscissa is the temperature T of resin and the ordinate the specific volume V, and the relation between V and T for the resin is shown for a constant applied pressure.
Using the aforesaid injection compression molding apparatus, one can trace on this graph the steps of injecting resin into the cavity of the mold, compressing it, and ejecting it therefrom. The terminal point V-P of the injection stage is represented by A, and the pressure of the resin is increased along the path A-B due to injection pressure while the temperature of resin is falling. The temperature of resin continues to fall even after the injection pressure is discontinued, and since the volume decreases even without external pressure and has the specific volume under low pressure, the path B-C is traced. At time C compression is applied by fully extending the toggle, while taking backward flow into consideration, and the pressure in the resin increases before the temperature of the resin falls very much, i.e. along the path C-D. At this time, while the toggle is extended to the fullest, the temperature of resin falls and the pressure also falls slightly along the path D-E as the volume is reduced by curing and cooling, and this causes some movement of resin. However, this pressure is applied to the resin while the fluidity is deteriorating, thereby generating strain within the resin. Thereafter, upon opening the mold at a temperature at which ejection is desired, since the external pressure decreases, the resin pressure moves along the path E-F and the specific volume increases. Thereafter the pressure moves along the path F-G for normal cooling of resin under the atmospheric pressure, thus completing the molding.
In such a case, the shrinkage ratio during molding is obtainable from the difference between the specific volumes at the points E and G. The compression method using the toggle adjusts pressure by positioning a toggle arm with a fixed arm length, but because the pressure is different at different temperatures of the mold, the temperature of a tie-bar, and the temperature of the mechanism for positioning the toggle arm, it is difficult to keep the parts stationary. Therefore, it is difficult to control the compression pressure. Referring to the PVT curve, it is not clear whether the termination of the toggle arm movement gives a pressure at point D or D', thereby making accurate pressure control impossible.
On the other hand, the direct compression method using a hydraulic cylinder can precisely adjust maximum compression pressure. When the compression pressure, as shown in FIG. 2, is controlled to fall so that the specific volume of the resin remains constant as the temperature of the resin drops, i.e. moves along line D-E of FIG. 2, the resin is not deformed at all during the process of curing, thereby creating no strain, which shows that it is possible to carry out molding having a fixed mold shrinkage ratio. In this case, the difference in volume between the cavity and the resin coincides with the difference in specific volume between the points D and G, and it is possible to control the volume of the cavity so that it remains constant, thereby making possible molding with a constant shrinkage ratio from molding cycle to molding cycle.